Search and Rescue Direction Finders

[prv]

To DF something you need a minimum of three directions to the signal source and the result is a cocked hat,

No - that's to plot a position. To Direction Find you only need one antenna array, which tells you the direction. And if that antenna array is mounted on something moveable (eg, a boat) then you can run down the bearing until you find the transmitter.

As AntarcticPilot #11 says, you will only get sporadic signal and there is no way you will have time to get a bearing using a directional antenna.

The kit I've seen has a ring of LEDs, and the one representing the relevant bearing lights up. Even if it's only lighting up intermittently, that's still going to show you which way to go. Rotate the boat until it's the one at the top of the dial that lights, and keep it there as you move forwards.

that type of antenna ain't gonna fit on the average yacht.

This, plus the cost, are the real issues. It would be possible to fit it to many yachts, but you'd have to be pretty committed to 121.5 DF to put up with the bulk and ugliness of something like this up your mast all the time:

Pete

 

[ribrage]

No - that's to plot a position. To Direction Find you only need one antenna array, which tells you the direction. And if that antenna array is mounted on something moveable (eg, a boat) then you can run down the bearing until you find the transmitter.



The kit I've seen has a ring of LEDs, and the one representing the relevant bearing lights up. Even if it's only lighting up intermittently, that's still going to show you which way to go. Rotate the boat until it's the one at the top of the dial that lights, and keep it there as you move forwards.



This, plus the cost, are the real issues. It would be possible to fit it to many yachts, but you'd have to be pretty committed to 121.5 DF to put up with the bulk and ugliness of something like this up your mast all the time:

Pete

This is the original kit we had, the problem was the complexity of use for differing crews, the operation at high speed is difficult, add into the equation that you had to fit additional base stations to the mother vessel, making it the only one that knew what was going on !...... PLUS the range was PANTS

http://mrtsos.com/products/base-units

I think some people are confusing localised MOB alarms which are basically proximity sensors. IE go out of range of a base station and the alarm goes off (ON THE SINGLE BASE STATION) with PLB's which trigger alerts with MRCC directly , the reason I favour the unit I highlighted (that we have just taken delivery of) is the number of ways it alerts EVERYBODY via a number of means (as I highlighted) .

DF is like a beta max video recorder in comparison , yes its useful to RNLI because they have it mounted permanently and know how to use it , but if the mass market were using modern technology instead of 1930's VHF radios then the DF gear wouldn't get used (as often) as the rescue services would have a pretty picture of a MOB waving his arms on a computer screen (via AIS), a DSC call on his radio with written lat and long (updated routinely) and a voice call on ch16 tell him all he needs to know - its information over load (almost)

you pays your money and you takes your choice

 

[prv]

I think some people are confusing localised MOB alarms which are basically proximity sensors. IE go out of range of a base station and the alarm goes off (ON THE SINGLE BASE STATION) with PLB's which trigger alerts with MRCC directly

I don't think anybody's mentioned simple MOB alarms (Raymarine Lifetag etc) in this thread. We're mostly comparing old-school DF of 121.5MHz (using as a transmitter the secondary local-homing signal of a PLB) with AIS SARTs.

DF is like a beta max video recorder in comparison

Agree. VHF DF is a useful tool in the CG / RNLI armoury, but 121.5 DF doesn't make sense any more for the rest of us.

As I said in the beginning, PLB for single-handers, AIS SART for crewed. And can someone please make a device that does both?

Pete

 

[ShinyShoe]

DF is like a beta max video recorder in comparison , yes its useful to RNLI because they have it mounted permanently and know how to use it , but if the mass market were using modern technology instead of 1930's VHF radios then the DF gear wouldn't get used (as often) as the rescue services would have a pretty picture of a MOB waving his arms on a computer screen (via AIS), a DSC call on his radio with written lat and long (updated routinely) and a voice call on ch16 tell him all he needs to know - its information over load (almost)

121.5 is an added extra as I see it on the sat signal that is doing all of that. There because original EPIRBs didn't have the ability to send a long/lat position so gave a 5km search zone every 90minutes or so. You turn up with your lifeboat in the 5km search zone and find the 121.5 signal and go hunting... With modern EPIRBS/PLBs including GPS you would generally expect an exact position, but bear in mind to be given that position it has to hit a satellite ping that to earth (if it hits an oribiting sat rather than a geostationary that adds a delay) then that has to be processed and sent to the lead MRCC for the country - Falmouth for us. Thats based on the origin of the MMSI no in the device the location of the emergency could be Australia! They then have to find the relevant MRCC and pass the details. If they've done that and an updated position comes through it should be quicker to do as positions update but they wont know the point that assets are arriving on scene and so someone may need to ask for a pos update. If you are a MOB in a location with a 2kt current and the first asset takes an hour to get on scene you'll be 2 NM away from the original pos when it arrives. They could then ask falmouth (via local MRCC) if there is an updated pos they get that and head 2 miles away. ALB's travel at about 20kts so that takes 6 minutes to do the journey, and say 3 minutes for the 'age' of the pos and its now 9 minutes old. (Yes I know MRCC will be running their drift predictions on that). 9 minutes is about 0.3NM which is still over half a kilometre away - they are getting closer to their 'prey' all the time but as we all know if they are looking for a head hopefully in a lifejacket but in reduce visibility because its dark, raining, wavy or foggy they wouldn't get a visual. 121.5 would let them know which direction to have people looking and to head off to...

There are cases of people with GPS based PLBs reporting delays of about 45mins to an hour before the system fully works, possibly due to delays getting a GPS fix in a moving device.

121.5 wouldn't be needed if the unit was sending live data to AIS instead. If for some reason I'm not sending a pos I'd quite like 121 to be working!!

Sending data to DSC pos's would still be intermittent but avoids all the delays of sat bouncing.

As I said in the beginning, PLB for single-handers, AIS SART for crewed. And can someone please make a device that does both?

If you are going to do AIS I think you could probably transmit a sentence on CH70 as well which increases the number of boats who may be able to help you although I do agree both would be good. And with DSC you could also have an acknowledgement sent to the MOB. I gather battery power is part of the issue. Everyone seems to have decided their PLB needs to work for 24hours. Personally in all but ocean crossings I'm hoping to have been rescued in 3 hours rather than 30 hours... and you need the battery to last in a dormant state for 5 years. A single DSC alert would take very little additional power, even repeating it 5 times at intervals like the GMDSS requires. I suspect 10second updates to AIS are a bit more power hungry over 24hours...

Its all a compromise...

 

[prv]

Everyone seems to have decided their PLB needs to work for 24hours.

I don't think it's the manufacturer's choice - minimum operating time is part of the definition of EPIRB or PLB (EPIRB has to run for at least 48 hours, PLB for 24) so I assume they can't get a device certified without that.

I suspect 10second updates to AIS are a bit more power hungry over 24hours...

I don't think the Kannad AIS SARTs transmit every ten seconds, though I can't find how often they do. The spec says they only update their position every minute, so perhaps that's when they transmit. I do know that they send a burst of identical messages in quick succession and then rest for a while - the idea is that during that burst (I think it lasts 8 seconds) you should pass over the crest of a wave and so some of the messages will get out to maximum range.

Pete

 

[chanelyacht]

I'm trying to determine if DF an MOB transmitting on 121.5MHz is any easier / better than locating an MOB transmitting an AIS signal.

For a start, aircraft (apart from the newer CG helos) don't carry AIS kit, so I know which one I'd want...

 

[ShinyShoe]

For a start, aircraft (apart from the newer CG helos) don't carry AIS kit, so I know which one I'd want...

Brings you back to who you are expecting to rescue your MOB. Yourself - you need DF kit but could have it. Another vessel - far more likely to carry AIS than DF. An RNLI lifeboat - carry both. A CG Helo - carry both. Other Aircraft 121. So if you are going very offshore with less shipping, possibility of aircraft spotter planes looking for you etc you want EPIRB to get the message out and 121 to find you. If you are staying in UK territorial waters and not single handed you could use AIS as in the next couple of years the CG Helos will replace all the MOD ones so you would have cover, and even if they didn't a local vessel could pass the AIS co-ords to the helo.

If you are picking up your own MOB I think everyone would say AIS is most logical. Only concern I'd have is that a PLB is a good alternative to an EPIRB (or addition) - AIS far less so as if you sink you aren't rescuing yourself.

 

[tudorsailor]

The system is company wide and in use all around the world, the ships have numerous flags of convenience around the world.

The site map says pretty much world wide

http://mrtsos.com/

And these are what we are updating too (AIS Upgrade) , I have a case full of them on board , just waiting for the correct pouches to mount them to our life jackets

http://mrtsos.com/products/sMRT-v100-vhf-dsc-ais-maritime-survivor-locating-device

What I like best is that the range is greatly increased and any vessel nearby also gets to know I'm in the water and come bloody find me ! No special kit required to become a SAR vessel , no additional base stations to fit to the rescue boats or workboats, Ill update you when Ive fitted and tested them if your interested ?

ian

The title of the alarm says AIS but the blurb does not mention AIS. Confusing

Any idea what these cost for individual's purchase in comparison to the AIS alarms which have to be manually activated?

TS

 

[ShinyShoe]

The title of the alarm says AIS but the blurb does not mention AIS. Confusing

Any idea what these cost for individual's purchase in comparison to the AIS alarms which have to be manually activated?

TS

http://mrtsos.com/products/sMRT-v100-vhf-dsc-ais-maritime-survivor-locating-device Mentions it numerous places. Sends DSC alert to the host craft MMSI. If not acknowledged* then repeats alert to ALL STATIONS. AIS updates pos every minute.

*acknowledged sounds like you might not be able to use Class D DSC Kit? You can normally only acknowledge a DSC Alert with Class A kit. The Alert sent is MOB so I can't see how you'd acknowledge it with a Class D. But I can see that its configured to work that way to mean that and false alarms can be acknowledged quickly without resulting in a load of assets being deployed...

Only fixes GPS every 5 mins for 30mins then every 10mins??

 

[prv]

in comparison to the AIS alarms which have to be manually activated?

The Kannad / McMurdo ones can be installed in a lifejacket in such a way that inflating the jacket activates the beacon. Assuming an auto jacket, that means an auto beacon. They say it requires "professional installation" and is only available with certain jackets, but having closely inspected various videos and photos of such installations I can't see any real basis for that. Essentially there's a piece of string which gets pulled to activate the beacon, and you attach that in such a way that the inflating bladder will pull it. Or if you really can't arrange that (and I guess with some very simple combined-bladder-and-cover jackets there may be no suitable attachment point) then there's a dangling orange tab which you pull so although it's manual it couldn't really be simpler.

Pete

 

[ShinyShoe]

Didn't realise you could do that to make them auto. Sounds sensible at first, but I actually know no-one who's MOBd and not been able to manually activate it, secondly they perform badly if not held at the right position and thirdly I know LOTS of people who have LJs pop on them in very wet conditions or when they step off the boat at the jetty and mis-judge.

Now if that results in a local DSC alert first which needs to be cancelled or it escalates thats not so bad - although I'm sure for the fall off a jetty people will be fussing about getting the person out the water, to be fair they at least have someone where they shouldn't be and I guess a PLB from a marina will be less likely to result in a helo launch than a PLB out at sea. If it goes directly to MRCC that feels like they'd get a lot of false alarms from accidental life-jacket inflations including some that may happen while the boat is unmanned.

 

[prv]

Didn't realise you could do that to make them auto.
[...]
I guess a PLB from a marina will be less likely to result in a helo launch than a PLB out at sea.

I'm not talking about PLBs, I'm talking about AIS SARTs. Specifically, the Kannad R10: http://www.kannadmarine.com/en/safelink-r10 (or its McMurdo brother).

That's not going to trigger helicopter call-outs and widespread panic, just a few beeping chart-plotters within a fairly small radius.

Pete

 

[chanelyacht]

I'm not talking about PLBs, I'm talking about AIS SARTs. Specifically, the Kannad R10: http://www.kannadmarine.com/en/safelink-r10 (or its McMurdo brother).

That's not going to trigger helicopter call-outs and widespread panic, just a few beeping chart-plotters within a fairly small radius.

Pete

Which in busy areas will trigger calls to MRCCs, (or CGOCs in the future ). But it will also help bring assistance, which can't be a bad thing. Just don't rely on it as the primary means of alerting CG.